This invention relates generally to an ion source, and more specifically to a filtered ion source for use in a variety of ion applications where efficient ion transport and macroparticle filtering is desired.
Ion sources are used in a variety of applications, from heat treatments to physical vapor deposition (xe2x80x9cPVDxe2x80x9d) of materials on substrates. Typically, the ion source material is consumed and this results in the transmission of undesirable chunks or droplets called macroparticles. Macroparticles are undesirable for nearly all ion source applications, especially those applications involving PVD.
Prior art PVD techniques using an ion source incorporate a deposition or coating chamber in which a xe2x80x9cplasmaxe2x80x9d of the coating material is produced and projected toward a substrate to be coated. Coatings applied to substrates and the shapes and materials of the substrates can vary widely, from decorative coatings on ceramic or pottery materials to circuit interconnection wiring paths on the surfaces of semiconductor chips, to wear-resistant coatings on cutting tools and bearing surfaces. Similarly, the physical nature and properties of the coating materials may vary widely, from conductive coatings to semiconductive coatings to those forming electrical insulators. Physical vapor deposition processes generally require evacuation of the deposition chamber prior to and maintenance of a negative pressure level during the deposition coating process. In an electric arc type ion source, after evacuation of the chamber, the typically solid sacrificial source material is acted upon by an electric arc that converts the solid source material into a vaporous plasma of coating material. Once converted into a plasma, a coating source material may be combined with reactive gasses or other elements within the chamber to form coating compounds and molecules prior to or during deposition thereof on substrate(s). The coating plasma typically includes atoms, molecules, ionized atoms and molecules, and agglomerates of molecules.
Frequently, PVD techniques using an electric arc are preferable over other deposition methods due to the production of copious numbers of ions. The production of a highly ionized plasma combined with the use of an electrically biased substrate, allows the arrival energy of the ions to be controlled during deposition, thereby providing for optimization of film properties such as stoichiometry, adhesion, density, and hardness. As an example, the hardness of hard carbon films deposited using cathodic arc evaporation have been shown to be approximately four times the hardness of magnetron-sputtered hard-carbon films, approaching the hardness of natural diamond. During operation, an arc-initiating trigger element is positioned proximate the cathode source and is positively biased with respect to the cathode. The trigger element is momentarily allowed to engage the surface of the cathode material, establishing a current flow path through the trigger and cathode. As the trigger element is removed from engagement with the cathode surface, an electric arc is struck and thereafter maintained between the cathode and the anode electrodes. The electric arc carries high electric current levels, typically ranging from 30 to several hundred amperes, and provides energy for vaporizing the coating source material.
Notwithstanding the noted benefits of PVD using a cathodic arc (hardness, density, adhesion, and stoichiometry), this deposition technique has been plagued with several problems such as the dislodging of undesirably large pieces of the coating material previously referred to herein as macroparticles. These chunks or droplets of source material lead to blemishes in the coatings and exclude unfiltered or poorly filtered cathodic-arc ion sources from use in applications requiring extremely smooth films such as optical coatings or computer hard dive protective over-coatings. A number of approaches have been advanced for the removal of macroparticles. Despite prior art efforts at eliminating the transmission of macroparticles, no macroparticle removal techniques of the prior art (commonly referred to as xe2x80x9cfilteringxe2x80x9d) are able to produce macroparticle-free coatings without compromising other system aspects such as deposition rate, deposition area, ion transmission efficiency, and/or uniformity in coating. For example, many macroparticle filters of the prior art seek to separate the desired ions of cathode material through plasma optical techniques. Principally, these approaches always reside in leading the ions into an area where the workpieces may be arranged that is not in direct view of the cathode. Some of these techniques use deflecting tubes that eliminate direct line-of-sight paths between workpieces and the cathode. In these systems, macroparticles are captured on baffles arranged on the inside surface of the deflecting tubes and the workpieces are arranged at an open end of the deflecting tube and line-of-sight is prevented by a 45xc2x0 to 90xc2x0 bend in the tube. These systems are undesirable for many applications because only the ions that are emitted substantially along the axis of the tube are utilized, while ions traveling in other directions are lost. Additionally, the inside diameter of the deflecting tube must be small enough to prevent line-of-sight between the cathode and workpieces, so that the open cross-sectional area for ion passage through the filter is limited. Additionally, a significant portion of the ions that do make it into the curvilinear filter are subsequently lost to the walls during their relatively long passage through the tube, as a result of turbulence in the plasma. Consequentially, curvilinear type filters have an ion transmission efficiency of at best 25 percent. xe2x80x9cIonized Plasma Vapor Deposition and Filtered Arc Deposition; Processes, Properties and Applicationsxe2x80x9d by P. J. Martin et al., J. Vac. Sci. Technol. A 17(4) July/August 1999. Additionally, the ions that do make it through curvilinear filters are distributed over a small and non-uniform area. For example, coating thickness variations can be as high as 15 to 100 percent over a deposition area of only 100 to 150 mm.
In addition, smaller macroparticles that are emitted substantially parallel to the deflecting tube can be reflected around the bend of the tube and through the filter to the workpieces. Electrostatic reflection from the walls of the deflecting tube has been shown to be a primary mechanism whereby macroparticles are transmitted through curvilinear-type filters. xe2x80x9cMacroparticle Distribution in a Quarter-Torus Plasma Duct of a Filtered Vacuum Arc Deposition System,xe2x80x9d by M. Keidar et al., J. Phys. D: Appl. Phys. Vol. 30 (1997). Entrapment in the plasma stream and mechanical bouncing can also contribute to macroparticle transmission. Transmission of macroparticles through a filter by any of these mechanisms is more likely to occur when the geometry of the filter allows macroparticles to be emitted substantially parallel to the filter surfaces and/or in substantially the same direction as the plasma stream.
In an effort to reduce the transmission of macroparticles through the filter, two curvilinear filters connected together to form an S-shaped filter have been used to double the effect, but a further reduction in deposition rate has been noticed. For example, when this S-shaped-type filter is used, a 6 percent ion transport efficiency has been realized. Additionally, the larger size and complexity of this S-shaped filter limits its commercial application. To summarize, low ion transport efficiency, low deposition rate, small deposition area, macroparticle transmission and poor coating uniformity limit the commercial usefulness of curvilinear-type macroparticle filters.
Another approach to macroparticle filtration is described in U.S. Pat. No. 4,452,686 to Axenov et al. This approach utilizes ions emitted along the axis of a rotationally symmetrical container and leads these ions by reflection at the wall of the container around an obstacle which collects macroparticles, then into a substantially macroparticle-free area wherein the workpieces are arranged. The area which may be utilized for arranging workpieces is restricted to that part of the cross-sectional area of the container which is also the umbra of the obstacle for keeping off the droplets. Although this area will increase with decreasing diameter of the cathode, only a few workpieces may be coated simultaneously. Additionally, only the ions that are emitted along the axis are utilized while ions traveling in other directions are lost. This reduces the ion transport efficiency. Furthermore, similar to the curvilinear-type filters, macroparticles only have to be reflected through a small angle to reach the workpieces and are emitted in substantially the same direction as the plasma stream, so that transmission of macroparticles through reflection or entrainment in the plasma stream can still occur.
Furthermore, an approach to the elimination of macroparticles using another rotationally symmetrical device is shown in U.S. Pat. No. 5,282,944 to Sanders et al. This approach utilizes the ions emitted radially out from a ring-shaped cathode, and leads these ions by reflection through curved trajectories past baffles which collect macroparticles. In this approach, the area available for coating multiple workpieces is limited by the restricted opening through the baffles that is required to eliminate the typically line-of-sight path of the macroparticles between the cathode and the workpieces. Furthermore, macroparticles can be reflected through a small angle and can reach the workpieces and degrade the coating.
Yet another macroparticle filtering system is described in U.S. Pat. No. 4,929,321, granted to Buhl. This filtering approach directs ions emitted from an axially located cathode through a coaxial duct that blocks line-of-sight between the cathode and the workpieces, then radially transmits ions to the workpieces located at the outer periphery of the vacuum chamber. The axial portion of the vacuum chamber is not usable in this system and thus, much of the volume of the vacuum chamber is wasted. Additionally, this filtering approach cannot be separated from the design of the vacuum chamber, and thus, this filter is not easily adapted to preexisting vacuum chambers. Macroparticles reflected through small angles can also be a problem in this system.
Thus, in view of the foregoing problems with existing techniques for filtering macroparticles, there exists a need for an improved ion source that more effectively filters macroparticles out of the ion stream without compromising ion transport efficiency, deposition rate, deposition area, and coating uniformity. Additionally, there exists a need for an improved ion source that effectively filters macroparticles and substantially eliminates transmission of macroparticles by electrostatic reflection or entrapment in the plasma stream.
In view of the problems with existing macroparticle filtering techniques described above, the present invention relates to an apparatus and method for use in combination with an electric arc vapor deposition system (or any other ion source application where macroparticle filtering is desired) that substantially eliminates macroparticle transmission to the deposition area without substantially compromising deposition rate, deposition area, ion transport efficiency, or uniformity of the coating. In one embodiment, a filtered ion source useable with the present invention includes first and second ends connected by walls, said first end being closed and said second end being open to an area wherein workpieces may be treated; a consumable electrode source of material, said consumable electrode source including an evaporation surface positioned to face said first closed end and a first magnet for guiding ions along curved trajectories leading from the evaporation surface to the area wherein workpieces may be treated. In the embodiment just summarized, substantially macroparticle-free films are produced without substantially compromising deposition rate, deposition area, ion transport efficiency and uniformity because the evaporation surface of the consumable cathode material faces away from the deposition area and the ions are thereby required to bend around curved trajectories to reach the area wherein workpieces may be treated, while undesirable macroparticles, which are not strongly affected by magnetic or electrostatic forces, are substantially prevented from reaching the workpieces.